This invention relates to an earth station for use in a time division multiple access (often abbreviated to TDMA) satellite communication network.
A TDMA satellite communication network comprises a plurality of earth stations communicable with one another through a geostationary satellite (hereafter referred to simply as a satellite) in a time division fashion. Such communication is carried out in each earth station by sending an up-link signal to the satellite and by receiving a down-link signal from the satellite. Each of the up-link and the down-link signals is divisible into a succession of frames. Each frame includes a plurality of time slots to which bursts are assigned by the respective earth stations.
In such a communication network, accurate synchronization must be established in relation to the frames and the bursts of each of the up-link and the down-link signals. Otherwise, overlap takes place between two adjacent ones of the bursts.
One of the earth stations is selected as a control or reference earth station for producing a succession of reference bursts which define the frames in the up-link signal sent from the control earth station and which may collectively be called a transmission reference timing signal. The reference bursts appear through the satellite in the down-link signal and are delivered to each earth station as a reception timing signal. The earth stations establish frame synchronization with reference to the reception timing signal and thereafter carry out reception and transmission. In addition, transmission timing is also determined in each earth station in relation to the reception timing signal.
On receiving a burst from the down-link signal within an assigned one of the down-link signal after establishment of the frame synchronization, each earth station detects a fixed pattern by monitoring autocorrelation between the fixed pattern and a received pattern included in a received one of the bursts. However, false or wrong detection of the fixed pattern often takes place due to thermal noise and the like.
In order to avoid such false detection, each earth station is controlled so as to intermittently open a window at a predicted time location at which a burst might be received. In this event, detection of the fixed pattern is valid only when the window is opened. As a result, superfluous or false detection of the fixed pattern becomes invalid in each earth station.
In the meantime, a distance is variable between the satellite and each earth station in the TDMA satellite communication because the satellite fluctuates from a standard position to a deviated position with time with a spatial deviation left between the standard and the deviated positions. It is therefore necessary to control or adjust synchronization of each earth station in consideration of a variation or deviation of the distance between the satellite and the earth station under consideration.
In a conventional TDMA satellite communication network, synchronization of each earth station is carried out so that the reception timing signal is received within the window by controlling the transmission timing.
More particularly, each earth station determines its own reception time slots with reference to the reference bursts to open the windows at the reception time locations. Under the circumstances, the transmission timing of each earth station is controlled so as to carry out reception within the windows. For example, it is assumed that a certain earth station can transmit its own burst to the satellite and receive the own burst in question as a reception burst. In this event, a time relationship between the reference burst and the own burst is monitored to control the transmission timing which might vary due to fluctuation of the satellite. On the other hand, when an earth station can not receive the own burst, the transmission timing is adjusted in the earth station in response to an information signal which is indicative of fluctuation of the satellite and which is given from any other earth station. Such an information signal may be either a position error signal representative of a difference between the standard position and the deviated one or a distance signal representative of a distance between the deviated position of the satellite and each earth station.
As mentioned before, each earth station must carry out operation so as to adjust synchronization and to correct fluctuation or variation of the satellite when the transmission timing is controlled in each earth station. Moreover, either the position error signal or the distance signal should be sent from any other station to the earth station when the earth station in question can not receive its own transmission burst. Thus, a superfluous information signal must be transmitted between the earth stations. Transmission of a superfluous information signal gives rise to a reduction of throughput in the satellite communication network.
Moreover, communication control is widely distributed to the local earth stations in addition to the control earth station. This makes concentration of communication control difficult.
At any rate, an expensive and complicated transmission timing control device must be included in each local earth station to control the transmission timing and results in an increase of costs of the local earth station and in an increase of a load in the local earth station.